Correlated double sampling is a technique of taking two samples of a signal closely spaced in time and subtracting the first signal from the second to remove low-frequency noise. Sampling of the pixel output occurs twice: once after reset and once after integrating the signal charge. The subtraction removes the reset noise (kTC noise) and dc offset from the signal charge.
For correlated double sampling in a sensor array, a pixel of the sensor array needs to be able to yield, just after each other, the reset voltage on a capacitance, and the signal voltage on the same capacitance after it has integrated charge. Consecutive electronic circuitry (which is straightforward for persons skilled in the art) subtracts these two voltages and yields a more offset-free and noise-free result. For this reason, prior art pixel structures are provided with at least one memory element for memorising the reset level on the photodiode. At the moment of readout of the pixels, this reset voltage can be read, and the actual signal voltage, possibly also memorised in a second memory element, can be read immediately after it.
In current sensor devices the time difference between taking the two samples per pixel period is of the order of 100 ns or smaller. The time needed to completely reset the pixel is technology dependent. The time needed to calculate the correlated double sampling is also technology dependent. In conventional pixels, the reset time of the pixel is extended and made at least as long as the time needed to calculate the correlated double sampling. This slows down the pixel operation.